There is a class of devices generally referred to as Variable Optical Attenuators (VOAs). A VOA is used to reduce the power of an optical signal so that the resulting power level is within the acceptable range of those devices or instruments working downstream from the VOA. For example, a VOA may be used to equalize the power levels of multiple optical signals before the signals are combined in a DWDM system (Dense Wavelength Division Multiplexing) for high-speed transport. This equalization is required because the multiplexed optical signals will be amplified before being transported an d y excessively high power signals could be lost due to saturation. VOAs may also be required after the signals are multiplexed in a DWDM system to reduce the output power, The reason is that the actual power is dependent on the number of active channels, which can vary over time.
A VOA is one of the key components used in fiber optic communication systems. During the past decade, the demand for higher bandwidth driven by the Internet has resulted in a need for mass-producible and low cost optical components. A successful strategy used to reduce cost is to design optical components by leveraging the well-established manufacturing processes taken from the semiconductor industry. A strong interest exists, therefore, to produce VOAs and other optical components from typical semiconductor materials such as silicon, silica, nitrite and others. New developments are also seeking to produce these components using active materials such as gallium arsenide because these materials can be used to produce light generating components. An ultimate goal is to integrate a maximum number of functions on a single substrate to minimize the manufacturing cost.
There are prior art methods for adjusting the output power of an optical signal. The most common way to adjust the power of an optical signal is by simply limiting the amount of light transmitted from one fiber to another fiber. This can be accomplished by inserting an object (optically opaque in the wavelength of interest) between the light-carrying fiber and the outgoing fiber. The optically opaque object, usually referred to as a shutter, can be moved in small distances such that the amount of light captured by the receiving fiber can be controlled precisely. Conventional VOAs move the shutter by using precise mechanical stages and motors that have resulted in large and expensive systems. Other techniques rely on optical properties of selective materials such as liquid crystals to affect the amount of light passing through the material. Electro-optics and thermo-optical effects have also been used to affect the amount of light transmitted.
More recently, it has been desirable to produce VOAs using materials and processes compatible with semiconductor manufacturing processes. FIG. 1 illustrates an example of a prior art approach where a miniature actuator 10 is fabricated directly on the silicon substrate 20. Light is conducted into the switching region by an optical fiber 22. A shutter 24 is positioned between the end of the input fiber 22 and the entrance of the output fiber 26. The shutter 24 is supported by an actuator/micro-mechanism 10 produced out of silicon. The actuator/micro-mechanism 10 moves the shutter in the direction indicated by the actuation arrow. Electrical interface pads 28 may be coupled to the actuator/micro-mechanism 10 in order to control the actuator/micro-mechanism 10. By moving the shutter 24, more or less of the light from the input optical fiber 22 can be allowed to pass into the output optical fiber 26. This approach is described in U.S. Pat. No. 6,173,105. A wide range of fabrication technologies referred to as MEMS processes (Micro-Electro Mechanical Systems) have been employed successfully to produce these micro-mechanisms. Different methods of actuation are available including electrostatic, thermal and magnetic. The use of MEMS technology allows precise control of the actuator/mechanism 10 as well as batch manufacturing processes.
One problem associated with a VOA based on the shutter approach is the difficulty of integrating it with optical systems that use waveguides. Waveguides, by contrast with shutters, are optically transmissive structures. In the typical semiconductor process, different layers of materials are sequentially deposited and patterned. In the shutter approach, the silicon shutter must be located on the same plane as the waveguides and also must be physically larger than the waveguides to provide effective blocking of light. These two requirements make it difficult to produce both shutter and waveguides in the same processing sequence. Although it is possible create the shutter and waveguides separately by breaking up the process and by selective masking, this approach increases potential misalignments and manufacturing complexity.
Ideally, a VOA design for integration with a waveguide-based system uses the same processing steps as that used to make waveguides. One choice is to introduce a mechanism into the waveguide that would modulate light. That can be achieved by introducing electro-optical, thermal, or acousto-optical effects into the waveguides. These methods, however, are limited to waveguides made out of certain active materials, which waveguides are generally difficult to manufacture. Another possibility is to use a waveguide with a movable section which acts as a shutter by doping the movable section of the waveguide so as to become opaque. However, all of these methods require significant deviations from standard waveguide manufacturing processes. Therefore, there is a need for a device that changes the optical intensity of an optical signal which uses standard waveguide manufacturing processes. There is also a need for a cost effective method of fabricating such a device.